--partition flag for clusterawsadm

[aaroniscode]

What this PR does / why we need it:
Currently, clusterawsadm can create IAM policies for the stardard AWS regions. For AWS GovCloud and AWS China, the ARN needs to updated with a specific partition name. From the AWS documentation:

AWS accounts are scoped to a single partition. You can get a partition by name. Valid partition names include:

"aws" - Public AWS partition
"aws-cn" - AWS China
"aws-us-gov" - AWS GovCloud

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #959

Special notes for your reviewer:
I'm not super familiar with Cobra, so let me know if there is a different convention I should use for setting the flag and retrieving it's value.

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

clusterawsadm: Support for different AWS partitions, including AWS GovCloud, using the --partition flag

[k8s-ci-robot]

Hi @aaroniscode. Thanks for your PR.

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[vincepri]

/approve
/ok-to-test
/assign @detiber

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aaroniscode, vincepri

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[vincepri]

We'd also need to backport this change to release-0.3 branch.

[aaroniscode]

We'd also need to backport this change to release-0.3 branch.

@vincepri I've got that ready. Do I just open a separate pull request on that branch?

[vincepri]

Correct!

[detiber]

/lgtm